1. Field of Invention
The present invention relates to a device for signal processing. More particularly, the present invention relates to a variable gain signal processing device.
2. Description of Related Art
In an image signal processing system, the strength of the signal received by the system is always changed considerably during transmission. Thus, the circuits with auto gain control (AGC) are usually used to automatically adjust the strength of signal received by the system to meet the system's input signal specification. A variable gain device, or referred to as variable gain amplifier (VGA), is indispensable in an AGC system.
To ensure the received signal meets various input signal specification requirements of the system, some VGA designs are widely used and discussed in the literature. Moreover, for ensuring that the system to have an ideal image output quality, the VGA has to have such characteristics as lower distortion and wider frequency range. Meanwhile, for overcoming the characteristic of the strength of the signal received by the system having wider range of variation, the VGA has to have wider gain range and higher linearity. Accordingly, a VGA having lower distortion, wider frequency range, and wider gain range has become a critical device in an image signal processing system.
FIG. 1 is a circuit diagram of a conventional VGA. Referring to FIG. 1, the conventional VGA includes 6 transistors 101, 102, 103, 104, 105, 106, an attenuation resistor network 113, a first output resistor 107, a second output resistor 108, a first current source 109, a second current source 110, a third current source 111, and a fourth current source 112. In which, the transistors 101 and 102 form an input differential pair, and the attenuation resistor network 113 includes two sets of attenuation resistors RA1˜RAn and RB1˜RBn, and 2(n+1) switches SA1˜SA(n+1) and SB1˜SB(n+1), where n is a positive integer.
The transistors 101, 103 and a plurality of attenuation resistors RA1˜RAx in the attenuation resistor network 113, and the transistors 102, 104, and a plurality of attenuation resistors RB1˜RBy in the attenuation resistor network 113 form a transduction feedback circuit, respectively, in which x and y are both positive integers smaller than n. In addition, the transistors 103, 105 and the transistors 104, 106 form a current gain stage, respectively. Since the transistors 101 and 102 are forcibly turned on with the current of the first current source 109 and the current of the second current source 110 respectively, therefore, the transistors 101 and 102 of the input terminal can be ideally regarded as a DC level shifter. Thus, the input voltage at the gates of the transistors 101 and 102 can be linearly transmitted to the sources of the transistors 101 and 102. The currents passing through the transistors 103 and 104 can be regarded as a combination of a DC component current and an AC component current. In which, the DC component current conducted by the transistor 103 is the current of the third current source 111 deducted with the current of the first current source 109; and the AC component current conducted by the transistor 103 is approximately equal to the input signal divided by a portion of the attenuation resistance in the attenuation resistor network 113. This portion of the attenuation resistance does not include the resistance of the foregoing transduction feedback circuit. The portion of switches SA1, SA2, . . . , SA(n+1) and SB1, SB2, . . . SB(n+1) are switched (only one pair of corresponding switches are turned on at one time, while the remaining switches is turned off) to change the gain of the amplifier, so that the AC component current is changed according to the attenuation resistance corresponding to each pair of switches SA1, SA2, . . . , SA(n+1) and SB1, SB2, . . . SB(n+1). Accordingly to the above effect, the changes of the AC component current are respectively transmitted to the first output resistor 107 and the second output resistor 108 through the current gain stage transistors 103, 105 and the transistors 104, 106, so as to obtain the required gain.
In a conventional circuit, the gain generated is determined by the attenuation resistance and the internal resistance of the switches when the switches are turned on. However, the internal resistance of the switches SA1, SA2, . . . , SA(n+1), SB1, SB2, . . . SB(n+1) when the switches are turned on changes along with the oscillation amplitude of the input signal; meanwhile, the internal resistance of each turned-on switch is different, and thus the output linearity is not adequate. In addition, when the specification of the VGA requires wider gain range and smaller unit of gain, the number of attenuation resistors and switches required will be excessively large and complicated, which results in the increase in manufacturing cost. The unit of gain is referred to the difference between the gains corresponding to two adjacent switches, and the difference between the gains of each two adjacent switches is approximately same since the VGA is a linear amplifier.